Method for manufacture of films and foams

ABSTRACT

The invention relates to a method for the manufacture of films from colloidal suspensions of two-dimensional materials. Specifically, the method involves controlled drying and dispensing of the colloidal suspension on a substrate. The resulting films are freestanding. The invention further relates to the formation of freestanding foams.

Field of Invention

The invention relates to a method for the manufacture of films from colloidal suspensions of two-dimensional materials. Specifically, the method involves controlled drying and dispensing of the colloidal suspension on a substrate. The resulting films are freestanding. The invention further relates to the formation of freestanding foams.

Background

Various materials can be used to form films or monolayers. These structures have many desirable properties and find use in a variety of industries. However, current methods of production, such as chemical vapour deposition (CVD), are often not well-suited to large scale manufacture and/or are prohibitively expensive. Current methods are also incapable of producing freestanding films or foams.

SUMMARY OF THE INVENTION

The present invention generally resides in a novel method of manufacturing freestanding films. Therefore, in a first aspect, the present invention provides a method for making a freestanding film, the method comprising: applying a colloidal suspension of a two-dimensional material to a substrate; and drying the solution to form the film on the substrate. The film is released from the substrate to form a freestanding film. Preferably, the substrate is a blue steel substrate or similar material.

Preferably, the method further comprises removing the film from the blue steel substrate to form a freestanding film.

In certain preferred embodiments the colloidal suspension is allowed to dry in air on the blue steel substrate. Preferably the colloidal suspension is allowed to dry in air for up to 32 hours. In other preferred embodiments, the blue steel substrate and colloidal suspension are dried using infrared radiation, preferably multi-frequency infrared radiation. In certain embodiments heated or pre-heated substrate may be used.

In order to regulate film formation, a bias voltage may be applied to the colloidal suspension and/or substrate during dispensing or drying.

The two-dimensional material may be any suitable two-dimensional material created from bulk 3D or higher dimensional material, i.e., any material capable for forming a film or monolayer. Preferably, the two-dimensional material is selected from the group comprising: group IV materials including graphite, graphite oxide, graphene (C), graphene oxide, graphane (CH), fluorographene (CF), silicene (Si), germanane (GeH), stanene (Sn), borophene (B), and thiophene; MXenes including Ti₃C₂, Ti₂C, TaZ₄C₃, and Ti₃(Co.5No.5)₂; dichalcogenides including VSe₂, NbSe₂, TiS₂, ZrS₂, HfS₂, ReS₂, PtS₂, TiSe₂, ZrSe₂, HfSe₂, ReSe₂, PtSe₂, SnSe₂, TiTe₂, ZrTe₂, VTe₂, NbTe₂, TaTe₂, MoTe₂, WTe₂, CoTe₂, RhTe₂, IrTe₂, NiTe₂, PdTe₂, PtTe₂, SiTe₂, NbS₂, TaS₂, MoS₂, WS₂, VS₂, TaSe₂, MoSe₂, WSe₂, MoTe₂, SnSe₂, and SnS₂; trichalcogenides including Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, Bi₂S₃, In₂Se₃, As₂S₃, As₂Se₃, NbSe₃, TiS₃, ZrS₃, ZrSe₃, ZrTe₃, HfS₃, HfSe₃, HfTe₃, NbS₃, TaS₃, and TaSe₃; mono-chalcogenides including GeSe, GeTe, GaSe, and GaS; thiophosphates including FePS₃, MnPS₃, and NiPS₃; oxides including MnO₂, δ-MnO₂, MoO₃, V₂O₅, WO₃, and TiO₂; nitrides including BN; oxychlorides including BiOCI, FeOCI, HoOCI, ErOCI, ErOCI, TmOCI, YbOCI, and LnOCI; halides including FeCI₃, FeBr₃, CrCI₃, CrBr₃, MoCI₃, MoBr₃, TiCI₂, TiBr₃, InBr₃, PbCI₂, AlCI₃3, InBr₃3, CrBr₃3, FeCI₂, MgCI₂, CoCI₂, VCI₂, VBr₂, Vl2 CdCI₂, CdCI₂; layered silicate minerals including calcium copper silicate (Egyptian Blue); and other materials including CaSi₂, CaGe₂, Ca(Si_(1-x)Ge_(x))₂, Ba₃Sn₄As₆, CaMg₂N₂, CaIn₂, CaNi₂P₂, CaAuGa, InAs, WS₂, TiSe₂, Bi₂Se₃, Sb₂Te₃, Bi₂Te₃, magnesium diboride, TBA_(x)H_((1.07-x))Ti_(1.73)O₄.H₂O, CoO₂—, TBA_(x)H_((1-x))Ca₂Nb₃O₁₀, Bi₂SrTa₂O₉, Cs₄W₁₁O₃₆ ²⁻, Ni(OH)_(5/3)DS_(1/3), Eu(OH)_(2.5)(DS)_(0.5), Co_(2/3)Fe_(1/3)(OH)2 ^(1/3+) and iron pentacarbonyl. Additional materials include Mn, Co, Cu, Zn, Ga, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, Bi, La, Eu, Gd, U, Pu, P, As, O, S, Se, Te, F, PdS, HgS, Ag2S, Cu2S, SnS, PbS, CdS, ZnS, NiS.

In certain preferred embodiments the two-dimensional material is not graphene or graphene oxide. In other preferred embodiments the two-dimensional material is not graphite or graphite oxide

In certain embodiments, the solution further comprises graphite, graphite oxide, graphene, graphene oxide or a combination thereof.

In a second aspect, the present invention provides a freestanding film made by the method according to the first aspect of the invention.

In a third aspect, the present invention provides a method for making a freestanding foam, the method comprising: applying a colloidal suspension of a two-dimensional material to a substrate; and cooling the colloidal suspension and substrate under vacuum to form the foam on the substrate. The foam is released from the substrate to form a freestanding foam. Preferably, the substrate is a blue steel substrate or similar material.

Preferably, the method further comprises removing the foam from the blue steel substrate to form a freestanding foam.

The two-dimensional material may be any suitable two-dimensional material as described above in relation to the first aspect of the invention.

In preferred embodiments the colloidal suspension and substrate are cooled in stages. Preferably, the colloidal suspension and substrate are cooled to a final temperature of about −190.0° C. More preferably, the colloidal suspension and substrate are cooled in stages at about −178.1° C., about −181.0° C., about −190.8° C., about −160.3° C., and about −190.0° C.

In yet a further aspect, the present invention provides a freestanding foam made by the method according to the third aspect of the invention.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1a shows a schematic of a colloidal suspension being dispensed onto a blue steel substrate by a dispenser.

FIG. 1b shows a schematic of a colloidal suspension being dispensed onto a blue steel substrate by a dispenser whilst simultaneously being charged with a positive or negative charge. The dispenser, dispenser rings, and substrate may be charged.

FIG. 2a shows a schematic of the process self organization of the solid particles in the colloidal suspension on the blue steel substrate.

FIG. 2b shows a schematic of the structure of the organised particles in the colloidal suspension.

FIG. 3 shows a schematic of the drying process.

FIG. 4 shows a schematic of the structure of the blue steel surface.

FIG. 5 shows a schematic of an uneven substrate surface.

FIG. 6 shows a schematic of a circular substrate material with either internal or external longitudinal peaks (A and B, respectively), or external lateral peaks (C).

FIG. 7 shows a schematic of an exemplary molding process.

FIG. 8 shows a schematic of a top-down view of a standard (A) and deformed (B) substrate, showing that the peak to peak distance should be maintained even when the substrate is deformed.

DETAILED DESCRIPTION

The method of formation of freestanding films according to the present invention comprises dispensing a colloidal suspension of a two-dimensional material onto a substrate, preferably blue steel or a material with a similar physical structure. The suspension is allowed to dry, resulting in the formation of a film on the substrate. In certain embodiments drying is controlled via the use of a partial vacuum and/or infrared radiation. In certain embodiments, a heated or pre-heated substrate may be used. The film is then removed from the substrate as a freestanding film and can be used as needed.

Blue steel is a steel alloy comprising nickel and chromium and has a spheroidized carbon structure and is available commercially from a number of manufacturers, including JFE. Blue steel is further described in U.S. Pat. No. 8,071,018 and U.S. 8,052,812, the contents of which are incorporated herein by reference. Without wishing to be bound by any particular theory, the inventors believe that using blue steel as a metal substrate on which to form the sheets assists in the release of the sheets formed on the substrate. It is believed that the physical properties of the blue steel result in the film formed on the substrate being automatically released from the substrate as vaporization occurs. It may also be possible to assist in the early release of a film from the substrate by adding additional agent(s) (i.e., ammonium, boron, boron nitride, nitrogen, iron, iron oxide, etc) to the substrate by pre-treatment and/or to the colloidal liquid or a combination thereof. This means that that the films formed start to release from the surface of the blue steel substrate without further treatment. Again, without wishing to be bound by theory, it is believed that as the colloidal suspension settles on the blue steel substrate several events are taking place: (a) colloidal suspension is collating (i.e., the particles or flakes in suspension are forming an ordered structure); (b) the solution is vaporising, (c) the film is settling upon the spheroidized structure of the blue steel substrate; (d) the spheroidized structure of the blue steel substrate is creating a uniform film on its surface; and (e) the film starts to separate from the substrate as the film solidifies into a freestanding film.

The term “two-dimensional material” as used herein is intended to refer to any material with a substantially two-dimensional molecular structure. These materials are highly anisotropic and capable of forming ordered monolayers the thickness of which is approximately that of a single molecule of the material. Examples of such two-dimensional materials exist for almost every element on the periodic table. These examples typically occur when a periodic element is converted from its elemental nature into an oxide (organic or metal), hydroxide and chalcogenides, nitride, sulphide and or metal-organic compound, etc. Examples include but are not limited to group IV materials including graphite, graphite oxide, graphene (C), graphene oxide, graphane (CH), fluorographene (CF), silicene (Si), germanane (GeH), stanene (Sn), borophene (B), and thiophene; MXenes including Ti₃C₂, Ti₂C, Ta₄C₃, and Ti₃(Co.5No.5)₂; dichalcogenides including VSe₂, NbSe₂, TiS₂, ZrS₂, HfS₂, ReS₂, PtS₂, TiSe₂, ZrSe₂, HfSe₂, ReSe₂, PtSe₂, SnSe₂, TiTe₂, ZrTe₂, VTe₂, NbTe₂, TaTe₂, MoTe₂, WTe₂, CoTe₂, RhTe₂, IrTe₂, NiTe₂, PdTe₂, PtTe₂, SiTe₂, NbS₂, TaS₂, MoS₂, WS₂, VS₂, TaSe₂, MoSe₂, WSe₂, MoTe₂, SnSe₂, and SnS₂; trichalcogenides including Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, Bi₂S₃, In₂Se₃, As₂S₃, As₂Se₃, NbSe₃, TiS₃, ZrS₃, ZrSe₃, ZrTe₃, HfS₃, HfSe₃, HfTe₃, NbS₃, TaS₃, and TaSe₃; mono-chalcogenides including GeSe, GeTe, GaSe, and GaS; thiophosphates including FePS₃, MnPS₃, and NiPS₃; oxides including MnO₂, δ-MnO₂, MoO₃, V₂O₅, WO₃, and TiO₂; nitrides including BN; oxychlorides including BiOCI, FeOCI, HoOCI, ErOCI, ErOCI, TmOCI, YbOCI, and LnOCI; halides including FeCI₃, FeBr₃, CrCI₃, CrBr₃, MoCI₃, MoBr₃, TiCI₂, TiBr₃, InBr₃, PbCI₂, AlCI₃3, InBr₃3, CrBr₃3, FeCI₂, MgCI₂, CoCI₂, VCI₂, VBr₂, VI2 CdCI₂, CdCI₂; layered silicate minerals including calcium copper silicate (Egyptian Blue); and other materials including CaSi₂, CaGe₂, Ca(Si_(1-x)Ge_(x))₂, Ba₃Sn₄As₆, CaMg₂N₂, CaIn₂, CaNi₂P₂, CaAuGa, InAs, WS₂, TiSe₂, Bi₂Se₃, Sb₂Te₃, Bi₂Te₃, magnesium diboride, TBA_(x)H_((1.07-x))Ti_(1.73)O₄.H₂O, CoO₂—, TBA_(x)H_((1-x))Ca₂Nb₃O₁₀, Bi₂SrTa₂O₉, Cs₄W₁₁O₃₆ ²⁻, Ni(OH)_(5/3)DS_(1/3), EU(OH)_(2.5)(DS)_(0.5), CO_(2/3)Fe_(1/3)(OH)₂ ^(1/3+) and iron pentacarbonyl. Additional materials include Mn, Co, Cu, Zn, Ga, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, Bi, La, Eu, Gd, U, Pu, P, As, O, S, Se, Te, F, PdS, HgS, Ag2S, Cu2S, SnS, PbS, CdS, ZnS, NiS. Additional materials include Mn, Co, Cu, Zn, Ga, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, Bi, La, Eu, Gd, U, Pu, P, As, O, S, Se, Te, F, PdS, HgS, Ag2S, Cu2S, SnS, PbS, CdS, ZnS, NiS. Other materials with the requisite two-dimensional structure will be known to the skilled person. Such materials are readily available from a number of commercial suppliers. Two-dimensional materials demonstrate unique physical properties, such as charge density waves, topological insulator behaviour, 2D electron gas physics, superconductivity, spontaneous magnetisation and anisotropic transport properties, that are not present in their bulk layered counterparts.

As the layers peel off the bulk material the inherent properties are altered. Monolayer and few-layer materials have unique electronic, physical, and structural properties not possessed by the bulk material. For example chalcogenides (i.e., MoS₂) approaching few-layer has higher mobility, undergo phase changes i.e., from indirect band gap to direct band gap semiconductor with photoluminescence, having possible uses for 2D transistors. While other 2D few-layer or monolayer materials demonstrate thermoelectric or topological insulator properties as the layers peel off.

The terms “film” and “sheet” are used interchangeably in the present application and should be considered equivalent. The present method can be used to form freestanding single or multiple layer sheets. The term “freestanding” is used to indicate that the films are not attached to or supported by another structure. Preferably the sheets comprise one to hundred layers. More preferably the sheets comprise one to twenty layers, more preferably one to ten layers, more preferably two to ten layers. Particularly preferred sheets have 1, 2, 3, 4 or 5 layers. Sheets comprising a single layer may also be referred to as monolayers. In certain embodiments, layers may be formed from different materials to create a composite material.

Exemplary sheets formed by the method of the invention have a thickness of about 1 nm to about 100 nm. Preferably the sheets can have a thickness of 1 to 50 nm and more preferably a thickness of 2 to 10 nm. Particularly preferred sheets have a thickness of 1 nm, 2 nm, 3 nm, 4 nm or 5 nm. The thickness of the sheets formed can depend on the number of layers that the sheets comprise. The thickness of the sheet produced by the method can be varied. Ways of varying the sheet thickness include controlling the formation of the droplet size of the solution applied to the blue steel substrate when using a drop casting technique to apply the solution and/or controlling the concentration of material in the solution.

A variety of solvents may be used to form the colloidal suspension, including but not limited to water, acetaldehyde, acetic acid, acetone, acetonitrile, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-butoxyethanol, butyric acid, diethanolamine, diethylenetriamine, dimethylformamide, dimethoxyethane, dimethyl sulfoxide, 1,4-dioxane, ethanol, ethylamine, ethylene glycol, formic acid, furfuryl alcohol, glycerol, methanol, methyl diethanolamine, methyl isocyanide, 1-propanol, 1,3-propanediol, 1,5-pentanediol, 2-propanol, propanoic acid, propylene glycol, pyridine, tetrahydrofuran, triethylene glycol, pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, chloroform, diethyl ether, dichloromethane (DCM), ethyl acetate, propylene carbonate, formic acid, n-butanol, nitromethane and mixtures thereof. The most preferred solvent is water.

Droplet size may be controlled using solution density, water purity, needle size and shape (dependent upon dispensing equipment), shaped and tuned electric fields, robot movement relative to dispensing action, output pressure from dispensing tip, distance of tip from substrate and combinations thereof. To achieve 2 to 3 atom layer thicknesses, a preferred concentration of about 2 mg of 2D material per 100 ml of H₂O may be used. To achieve thicker layers the solution is adjusted to between 3 mg to 6 mg of 2D material per 100 ml of H₂O. About 0.1 to 1.5 mg of 2D material per 100 ml H₂O will be required to achieve a single atom layer.

Typically, the present method involves the use of flakes of a two-dimensional material suspended in a fluid to form a colloidal suspension. There are many methods of obtaining such flakes that typically fall into four categories: micromechanical (friction based methods); in situ growth (chemical vapour deposition based methods); lithium-intercalation/deintercalation; and exfoliation into colloidal solutions (liquid based methods).

In colloidal solutions the resulting flakes tend to be large; in the range of about 1 μm to about 60 μm, sometimes even larger. Smaller flakes, down to the nanometre scale, may be obtained with further processing. A variety of methods (e.g., centrifuges, filters, etc.) may be used to further discriminate and/or sort the size of flakes into the desired sizes. This can be used to ensure the isolation of lateral layers and/or domain sizes as desired. This process is desirable to improve quality, but it is not essential to the formation of sheets.

The flake generation process involves the oxidation of an elemental material, resulting in repulsive forces which are then used to exfoliate the oxide into many small single pieces (flakes, sometimes referred to as 2D flakes). An intercalator (such as, but not limited to, ammonia or ethylene glycol) may be used to assist in the separation of layers and to help prevent the recombination of layers back into a bulk material. In the case of metal oxides (tantalum, cobalt, manganese, perovskite, etc), protonation results in electrostatic repulsion forces that facilitate exfoliation. In these cases a commonly used intercalator is also used as described above, typically tetrabutylammonnium (TBA) or tetramethylammonium (TMA) and others.

In the case of hydroxides (e.g., nickel hydroxide), agents such as dodecyl sulcate (DS) ions are used with Ni(OH)₂ under reflux in formamide. A further extension to this method is to add sonication to the already weakened hydroxide and split the layers. It has been reported that with various types of intercalators brute force sonication can work.

In some cases a direct synthesis can also be applied such as in the case of simple precursors Fe(CO)₅ and the elemental chalcogens (Se and Te), in the presence of a surfactant creates single layers of PbO-type Beta-FeSe and Fe(Se,Te)⁵⁰.

Consider the experiment of making a MoS₂ film using the present process. Bulk MoS₂ will be prepared by solvothermal treatment of (NH₄)₂MoS₄ dissolved in methanol in the presence of poly(vinylpyrrolidone) and N₂H₄.H₂O (50%) as a reductant. Typically (NH₄)₂MoS₄ (approximately 2.5 to 12.5 μmol) is dissolved in methanol (25 ml) containing N₂H₄.H₂O (approximately 0.02 to 0.1 ml) and an appropriate amount of poly(vinylpyrrolidone). The solution is then added to a Teflon lined autoclave and heated at temperatures of from about 373 K to about 473 K for around 3 hours to obtain colloidal MoS₂. The resulting MoS₂ is then placed into a solution of acetonitrile and H₂O to form a homogenous solution.

Another method is to fabricate reduced graphene oxide (RGO)/MnO₂ hybrids for ultrafast oxidative decomposition of methylene blue (MB) dye. First, pristine 10 ml of GO/MnSO₄ suspension (13 mg ml⁻¹ GO and 0.22 mol I⁻¹ MnSO₄) was produced by the modified Hummers method (Hummers et al, Journal of the American Chemical Society, 1958, vol. 80 p1339) using graphite, KMnO₄ and concentrated H₂SO₄ etc. as the precursors. In this process, large quantities of produced Mn (II) ions bound with the O atoms of the negatively charged oxygen-containing functional groups on GO sheets by an electrostatic force. To transform Mn²⁺ into MnO₂, more KMnO₄ is added slowly into the mix. Thus, highly dispersed MnO₂ nanoparticles are in situ deposited and anchored onto the surfaces of the GO to form GO/MnO₂ composites. Lastly, RGO/MnO₂ composites were fabricated by glucose-reduction of GO/MnO₂ composites and used as catalysts for decomposition of MB in the presence of H₂O₂.

It will be appreciated that many other examples of flake creation exist (such as plasma method, and sonication) which will be known to the skilled person. The methods described above are therefore merely exemplary and should not to be considered limiting on the present invention.

The next step is to dispense the colloidal liquid on the substrate, dry the material as described above and further process the resulting sheet from oxide to an elemental or functionalise the sheet in some way or remove the freestanding oxide sheet for use.

The present method starts with the dispensing (FIG. 1a ) of a colloidal solution containing the flakes to be rendered into a sheet. Dispensing may also include charging the dispenser with a positive or negative charge (FIG. 1b ) in order to impart a positive or negative charge on the solution, which may assist in nucleating the flakes. Various methods of liquid dispensing are known to the skilled person, such as for example methods used in ink-jet printers and in automated pipetting systems. Any suitable liquid dispensing method may be employed in the present invention. Where the liquid used is water, it is preferably of the highest quality (1,000,000 ohm cm resistivity or higher).

As the flakes settle (FIG. 2a ) on the substrate they laterally self-organize into an ordered shape (FIG. 2b ). Upon flakes setting a method of drying (FIG. 3) is utilised. Any suitable method of drying may be employed, including but not limited to: 1. pure vacuum drying, 2. partial vacuum drying with assistance of far infrared, short-wave infrared, medium wave infrared, microwave or a combination thereof. In certain embodiments, a heated or pre-heated substrate may be used. Optionally crystals or other substances that absorb water vapour from the evaporation process may be used during drying. Drying may also take place by adding nitrogen, argon or other gases, which is especially useful if further processing of the material (to reduce the material from an oxide) is intended.

Preferably the sheet is allowed to dry on the blue steel substrate for up to 32 hours if drying in air. Preferably the sheets can be dried for up to 20 to 24 hours in air. Preferably the sheets are dried at room temperature or up to 35° C. if drying in air. A pure or partial vacuum expedites the process of drying and lowers the temperature required to dry the material thus not damaging the material formation or its surface and also allowing for lower temperature two-dimensional materials to be processed safely. Using these processes drying over large surfaces (i.e., 1 meter by 1 meter, etc.) can be accomplished in minutes. Slow drying is recommended using this method so as not to cause bubbling in the solution which dislocates the graphene or graphene oxide crystals causing an uneven surface to be created.

Although simple, air-drying can cause inconsistency in the material due to thermal variations and uncontrolled shrinkage. Other drying methods can be used to expedite the drying process.

In particularly preferred embodiments drying is carried out via the use of infrared in combination with a low vacuum. This helps to create an environment in which “bubbling” of the solution does not occur, or where it is at least reduced to a point where it does not adversely affect the required properties of the final sheet material. Multiple infrared frequencies may be used to evenly heat and vaporise the water from the dispensed solution. In a particularly preferred embodiment drying is achieved using multi-frequency infrared radiation in a vacuum or in nitrogen gas. The drying method involves applying far, medium and short infrared frequencies with power in the range of 500 to 100 watts for 50 to 500 ns. When carried out under vacuum, the vacuum pressure is 3 kPa to 100 mPa. Alternatively, the method can be carried out under flow of dry nitrogen gas at 10 torr (1333 Pa), 50 standard cubic centimetres per minute (sccm). Water vapour is captured in the gas phase by vapour absorbing materials, such as hydrogel crystals. A pulsed mode device that uses a mixture of far, middle and short infrared radiation may be used to quickly dry the materials within a short time frame, which is dependent upon the surface area of the sheet being created, without the creation of bubbling. With this technique it is possible to dry out the film within a few seconds to minutes giving the same results as air-drying but without the time factor and without a considerable shrinkage of the sheet material.

The structure of the blue steel allows the film to be easily removed from its surface upon drying. The separation of the sheets from the blue steel substrate upon drying is due in part to the huge differences in expansion coefficient of the solution being much larger than that of the blue steel substrate. Blue steel has an especially small anisotropic number, i.e., it is directionally dependent (FIG. 4) on the sphereoidzed grain boundary in such a way as to have an extremely low expansion or contraction over a temperature range as high as 1000° C. Where also said sphereoidzed grain boundary is made in such a way that the surface at the nanometre scale is made up an evenly distributed (FIG. 4) boundary so that the flakes of 2D material will settle on these peaks with very little surface area contact of the flakes touching the surface. The flakes can therefore flow and self organise well and upon drying then promptly releases from the surface. It is noticed, after much testing of many other materials, that even on a normal sheet of highly polished metal, plastic or other materials that the surface is uneven and cratered as shown in FIG. 5.

Without wishing to be bound by any one particular theory, the present inventors believe that the peak to peak boundary as discussed above decreases the surface tension between the flakes and the substrate substantially. This assists in being able to remove the sheet for subsequent uses. It should be noted here that removal implies that as the sheet dries it mostly self releases on its own. However there are times when one must add extra force in the form of a peeling motion. It is known that this is because at the node where additional force is required for release there are defects in the substrate material. The structure of the blue steel can also assist in creating a uniform freestanding sheet on the substrate.

Once removed from the substrate, the freestanding sheets may be useful in a number of contexts. In certain embodiments, the now freestanding sheets may be applied to other surfaces or incorporated into other materials. In other preferred embodiments, various different sheets may be combined to form composites.

In the foregoing the production of flat sheet surfaces has been discussed. It is also mentioned here that the substrate material may be bent so as to be used on circular machinery (e.g., roll-to-roll, roll printing, as in capacitor, solar cell, news paper industries, tumbler, spin casting, spin melt, and centrifugal devices, wet spinning, dry spinning, electrospinning, etc), either internally (FIG. 6a ) or externally (FIG. 6b ) or both if so desired. Circular machinery may include (but is not limited to) drums, tubes, and conveyor belts. A large number of geometries may be used as long as the grain boundaries and peak to peak alignments are not tampered with.

Furthermore, other interesting applications of our invention include molding (FIG. 7) processes. This is accomplished by taking care not to damage the grain boundaries and peak to peak alignment when deforming (FIG. 8) the substrate material into the desired shape. Once the desired shape has been obtained then application of material can be handled in two ways, 1. deposition, 2. injection molding. Both of these methods will yield a net shape or near net (depending upon shrinkage or expansion of sheet which is dependent upon material properties) shape molded material that is then easily released from the substrate as a freestanding molded part.

In a third aspect, the present invention provides a method for making a freestanding foam, the method comprising: applying a colloidal suspension of a two-dimensional material to a substrate; and cooling the colloidal suspension and substrate under vacuum to form the foam on the substrate. The foam is released from the substrate to form a freestanding foam. Preferably, the substrate is a blue steel substrate or similar material.

Preferably, the method further comprises removing the foam from the blue steel substrate to form a freestanding foam.

The two-dimensional material may be any suitable two-dimensional material as described above in relation to the first aspect of the invention.

In order to form a freestanding foam, the substrate was placed in a vacuum chamber and 5 mL of graphene or graphene oxide was placed on substrate. Then vacuum pumps (such as molecular turbine type pumps) were used to evacuate the chamber to very high vacuum. The chamber was then cooled in stages using liquid nitrogen in the chamber. The first stage of cooling down was at −178.1° C. temperature, then the chamber was cooled to the next temperature stage at −181.0° C. temperature, followed by cooling to the next stage at −190.8° C. temperature. The temperature of the chamber was then raised to −160.3° C. temperature before being cooled to the final temperature of −190.0° C. The temperature was slowly brought back up to room temperature and the vacuum released. A raised liquid was observed on the substrate, which transformed itself into a foam material within a couple of minutes. This material peeled off the surface of the substrate into a rubber-like freestanding foam. The experiment took 4 hours.

The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and accompanying figures. Such modifications are intended to fall within the scope of the appended claims. Moreover, all embodiments described herein are considered to be broadly applicable and combinable with any and all other consistent embodiments, as appropriate.

Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties. 

1. A method for making a freestanding film, the method comprising: applying a colloidal suspension of a two-dimensional material to a substrate; and drying the solution to form the film on the substrate.
 2. The method according to claim 1, wherein the substrate is a blue steel substrate.
 3. The method according to claim 2, wherein the method further comprises removing the freestanding film from the blue steel substrate.
 4. The method according to claim 1, wherein the substrate and colloidal suspension is allowed to dry in air for up to 32 hours.
 5. The method according to claim 1, wherein the substrate and colloidal suspension are dried using infrared radiation.
 6. The method according to claim 1, wherein a bias voltage is applied to the colloidal suspension and/or substrate during dispensing.
 7. The method according to claim 1, wherein the two-dimensional material is selected from the group consisting of: group IV materials including graphite, graphite oxide, graphene (C), graphene oxide, graphane (CH), fluorographene (CF), silicene (Si), germanane (GeH), stanene (Sn), borophene (B) and thiophene; MXenes including Ti₃C₂, Ti₂C, Ta₄C₃, and Ti₃(Co.5No.5)₂; dichalcogenides including VSe₂, NbSe₂, TiS₂, ZrS₂, HfS₂, ReS₂, PtS₂, TiSe₂, ZrSe₂, HfSe₂, ReSe₂, PtSe₂, SnSe₂, TiTe₂, ZrTe₂, VTe₂, NbTe₂, TaTe₂, MoTe₂, WTe₂, CoTe₂, RhTe₂, IrTe₂, NiTe₂, PdTe₂, PtTe₂, SiTe₂, NbS₂, TaS₂, MoS₂, WS₂, VS₂, TaSe₂, MoSe₂, WSe₂, MoTe₂, SnSe₂, and SnS₂; trichalcogenides including Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, Bi₂S₃, In₂Se₃, As₂S₃, AS₂Se₃, NbSe₃, TiS₃, ZrS₃, ZrSe₃, ZrTe₃, HfS₃, HfSe₃, HfTe₃, NbS₃, TaS₃, and TaSe₃; mono-chalcogenides including GeSe, GeTe, GaSe, and GaS; thiophosphates including FePS₃, MnPS₃, and NiPS₃; oxides including MnO₂, δ-MnO₂, MoO₃, V₂O₅, WO₃, and TiO₂; nitrides including BN; oxychlorides including BiOCI, FeOCI, HoOCI, ErOCI, ErOCI, TmOCI, YbOCI, and LnOCI; halides including FeCI₃, FeBr₃, CrCI₃, CrBr₃, MoCI₃, MoBr₃, TiCI₂, TiBr₃, InBr₃, PbCI₂, AICI₃₃, InBr₃₃, CrBr₃₃, FeCI₂, MgCI₂, CoCI₂, VCI₂, VBr₂, VI₂ CdCI₂, CdOI₂; layered silicate minerals including calcium copper silicate (Egyptian Blue); and other materials including CaSi₂, CaGe₂, Ca(Si_(1-x)Ge_(x))₂, Ba₃Sn₄As₆, CaMg₂N₂, Caln₂, CaNi₂P₂, CaAuGa, InAs, WS₂, TiSe₂, Bi₂Se₃, Sb₂Te₃, Bi₂Te₃, magnesium diboride, TBA_(x)H(1.07_(-x))Ti_(1.73)O₄.H₂O, CoO₂−, TBA_(x)H(_(1-x))Ca₂Nb₃O₁₀, Bi₂SrTa₂O₉, Cs₄W₁₁O₃₆ ²⁻, Ni(OH)_(5/3)DS_(1/3), Eu(OH)_(2.5)(DS)_(0.5), CO_(2/3)Fe_(1/3)(OH)₂ ^(1/3+) and iron pentacarbonyl.
 8. The method according to claim 1, wherein the solution further comprises graphite, graphite oxide, graphene or graphene oxide.
 9. A freestanding film made by the method according to claim
 1. 10. A method for making a freestanding foam, the method comprising: applying a colloidal suspension of a two-dimensional material to a substrate; and cooling the colloidal suspension and substrate under vacuum to form the foam on the substrate.
 11. The method according to claim 10, wherein the substrate is a blue steel substrate.
 12. The method according to claim 11, wherein the method further comprises removing the freestanding foam from the blue steel substrate.
 13. The method according to claim 10, wherein the two-dimensional material is selected from the group consisting of: group IV materials including graphite, graphite oxide, graphene (C), graphene oxide, graphane (CH), fluorographene (CF), silicene (Si), germanane (GeH), stanene (Sn), borophene (B) and thiophene; MXenes including Ti₃C₂, Ti₂C, Ta4C₃, and Ti₃(Co.5No.5)₂; dichalcogenides including VSe₂, NbSe₂, TiS₂, ZrS₂, HfS₂, ReS₂, PtS₂, TiSe₂, ZrSe₂, HfSe₂, ReSe₂, PtSe₂, SnSe₂, TiTe₂, ZrTe₂, VTe₂, NbTe₂, TaTe₂, MoTe₂, WTe₂, CoTe₂, RhTe₂, IrTe₂, NiTe₂, PdTe₂, PtTe₂, SiTe₂, NbS₂, TaS₂, MoS₂, WS₂, VS₂, TaSe₂, MoSe₂, WSe₂, MoTe₂, SnSe₂, and SnS₂; trichalcogenides including Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, Bi₂S₃, In₂Se₃, AS₂S₃, As₂Se₃, NbSe₃, TiS₃, ZrS₃, ZrSe₃, ZrTe₃, HfS₃, HfSe₃, HfTe₃, NbS₃, TaS₃, and TaSe₃; mono-chalcogenides including GeSe, GeTe, GaSe, and GaS; thiophosphates including FePS₃, MnPS₃, and NiPS₃; oxides including MnO₂, 6-MnO₂, MoO₃, V₂O₅, WO₃, and TiO₂; nitrides including BN; oxychlorides including BiOCI, FeOCI, HoOCI, ErOCI, ErOCI, TmOCI, YbOCI, and LnOCI; halides including FeCI₃, FeBr₃, CrCI₃, CrBr₃, MoCI₃, MoBr₃, TiC1 ₂, TiBr₃, InBr₃, PbCl₂, AlCI₃3, InBr₃3, CrBr₃3, FeCI₂, MgCI₂, CoCI₂, VCI₂, VBr₂, VI₂ CdCI₂, CdCI₂; layered silicate minerals including calcium copper silicate (Egyptian Blue); and other materials including CaSi₂, CaGe₂, Ca(Si_(1-x)Ge_(x))₂, Ba₃Sn₄As₆, CaMg₂N₂, CaIn₂, CaNi₂P₂, CaAuGa, InAs, WS₂, TiSe₂, Bi₂Se₃, Sb₂Te₃, Bi₂Te₃, magnesium diboride, TBA_(x)H(_(1.07-x))Ti_(1.73)O₄.H₂O, CoO₂—, TBA_(x)H(_(1-x))Ca₂Nb₃O₁₀, Bi₂SrTa₂O₉, Cs₄W₁₁O₃₆ ²⁻, Ni(OH)_(5/3)D5 _(1/3), Eu(OH)_(2.5)(DS)_(0.5), CO_(2/3)Fe_(1/3)(OH )₂ ^(1/3+) and iron pentacarbonyl.
 14. The method according to claim 10, wherein the colloidal suspension and substrate are cooled in stages at about −178.1° C., about −181.0° C., about −190.8° C., about −160.3° C., and about −190.0° C.
 15. A freestanding foam made by the method according to claim
 10. 